In order to increase the resolution in capturing a large object, a plurality of overlapping partial images are taken and are composed into a single image. In the process of composing the overlapping partial images, one exemplary prior art reference, Hei 8-116490 discloses a system for taking a plurality of partially overlapping images with a predetermined number of cameras. The plurality of the images is corrected for their distortion and parallax. The corrected images are moved and rotated so that they are composed into a single image. However, it is impossible to perfectly match these partially overlapping images by moving and rotating since the direction of the image plane for these images is not identical when these images have been taken by an ordinary camera. In order to match the partially overlapping images without any disparity, it is necessary to perform perspective transformation on the images. The relation between a point on two image planes is as follows in Equation [1]:                               [                                                    x1                                                                    y1                                              ]                =                  [                                                                                          c0x0                    +                    c1y0                    +                    c2                                                        c6x0                    +                    c7y0                    +                    1                                                                                                                                            c3x0                    +                    c4y0                    +                    c5                                                        c6x0                    +                    c7y0                    +                    1                                                                                ]                                    [        1        ]            where the two points are (x0, y0) and (x1, y1) and c0 through c7 are variable parameters.
One prior attempt to use the above Equation [1] is “Video Mosaics for Virtual Environments” by Richard Szeliski, IEEE Computer Graphics and Applications, pp 22-30, March (1996). Without dividing an image into blocks, pixel values of two partially overlapping images are compared for a difference, and the parameters c0 through c7 of the Equation [1] are repeatedly evaluated by minimizing the difference. However, since the parameter value changes are determined by “yamanobori” based upon intensity gradient, depending upon the initialization of the parameter values, an erroneous minimization of the pixel value difference may follow. Thus, the correct solution is not necessarily obtained. Furthermore, when the light intensity is not uniform on an image, the above described minimal pixel value difference method cannot accommodate the light intensity variations. For example, a central portion of an image has less light intensity than its edge portions. Another example is that the light intensity of the image depends upon an angle of an image-capturing device, the location within the image or an automatic exposure function. Although, one way to improve the above problem is to reiterate non-linear optimized calculation, it is difficult to determine certain coefficients for the calculation and it is also time consuming to process the calculation.
Because of the above described difficulties, it has remained desired to improve a method of composing a single image from partially overlapping images that have been taken by a common image-capturing device.